The present invention relates to a fiber-optic amplifier.
The publication, Proc. ECOC 88, Brighton, pages 49 et seq., discloses that optical fibers which are doped with one or more lanthanoids, such as, for example, neodymium, erbium, or ytterbium, and which are based on SiO.sub.2 or fluoroid glass, when excited by light of a certain wavelength, act as an amplifier independently of the polarization of the exciting light wave. According to Proc. ECOC 88, Brighton, pages 58 et seq., an SiO.sub.2 fiber doped with Er.sup.3+ was fed with light signals through a directional coupler and connected with pump light sources of different pumping powers. It was then found that the degree of gain and a maximum gain wavelength in a range between 1520 and 1560 nm is a function of the pumping power of the pump light source. An 870 nm semiconductor laser was employed as the laser light source.
There are also reports that the wavelength at which maximum gain occurs in a fiber piece likewise changes as a function of the doping of the fiber piece.
In fiber-optic amplifiers of the above-mentioned type employed experimentally in the past, the gain was a function of the wavelength of the light signals, with the range of maximum gain being relatively narrow-banded. The gain decreases exponentially on both sides of this narrow band range of maximum gain as indicated in the second reference mentioned above. However, in glass fiber transmission systems operating with intensity modulation and direct detection, only the most broadbanded transmission function with the slightest possible ripple is permissible.